A traditional measurement that is necessary for safe operation of a motor vehicle is tire pressure. With the use of electronics it is possible to measure the pressure of each tire and provide continuous monitoring of the pressure to a driver of the vehicle.
Such a system generally includes a controller mounted on the motor vehicle and remote circuitry located in each tire for detecting the tire pressure, and transmitting the detected tire pressure to the controller. The controller receives the detected tire pressure and presents it to the driver.
As will be appreciated, providing power for, and communicating with, the remote circuitry in each tire of the vehicle can present considerable difficulty.
A known method of accomplishing this is to have a battery located in each tire to provide power for the remote circuitry, and to have a radio transmitter for transmitting the detected tire pressure using a radio frequency signal. This method was developed by Epic Technologies and disclosed on page 59 of an article in Engineering News published on Oct. 9, 1995. However, with this method replacement of the batteries is inconvenient, and in addition because of the high temperature and humidity in which the battery has to operate, the efficiency of the battery is adversely affected.
Another known method is to have a transponder mounted in each tire. An on-board antenna mounted on the vehicle body, close to each tire, radiates a radio frequency (RF) signal which provides sufficient power to the transponder for it to transmit an encoded signal. The encoded signal is received by an on-board controller and decoded. The decoded information identifies which tire on the vehicle transmitted the decoded signal, and indicates the detected, tire pressure in that tire. This method is disclosed in U.S. Pat. No. 5,235,850 assigned to Texas Instruments Inc.
While this method overcomes the disadvantages of using a battery, a disadvantage of this method is that the transponder requires complex circuitry to encode identification information and tire pressure data prior to transmission of the encoded signal. Consequently, due to the complexity of the circuitry required in each tire this method is expensive to implement.
In addition, dependent on the strength of the RF signal transmitted by the on-board antenna and the distance between the on-board antenna and the transponder in a tire, the power provided to the transponder will vary. Consequently, the strength of the signal transmitted by the transponder can also vary, which can affect the reliability of the operation of the transponder.
When the vehicle is in motion, the rotating transponder in the tire can come within range of the on-board antenna for sufficient periods to have reliable transponder operation. However, when the vehicle is stationary, dependent on the stationary position of the transponder in the tire, the variations in signal strength could cause unreliable transponder operation. This in turn leads to unreliable tire pressure detection.